I/O interface concept
An I/O interface is an electronic circuit (appearing in the form of an IC chip or interface board) containing several dedicated registers and corresponding control logic circuits. It serves as the medium and bridge for exchanging information between the CPU and I/O devices. Connections and data exchange between the CPU and external devices and memory all require interface devices; the former is called the I/O interface, and the latter is called the memory interface. Memory typically operates under the synchronous control of the CPU, and its interface circuit is relatively simple; however, there are many types of I/O devices, and their corresponding interface circuits vary. Therefore, conventionally, when people talk about interfaces, they usually mean I/O interfaces.
Basic functions of I/O interface
(1) Select a port address decoding device.
(2) Provide the CPU with I/O device status information and perform command decoding.
(3) Perform timing and corresponding timing control.
(4) Provide buffering for transmitted data to eliminate the difference in timing or data processing speed between the computer and peripherals.
(5) Provide compatibility conversion of information formats between the computer and peripherals. Provide electrical adaptation.
(6) Information exchange between the CPU and peripherals can also be achieved through interrupts.
Control method
(1) Program query method
In this method, the CPU queries the current status of the specified peripheral device through I/O instructions. If the peripheral device is ready, it performs data input or output; otherwise, the CPU waits and queries in a loop.
The advantage of this approach is its simple structure, requiring only a small amount of hardware circuitry. The disadvantage is that because the CPU is much faster than peripherals, it is usually in a waiting state, resulting in very low efficiency.
(2) Interruption handling method
In this way, the CPU no longer waits passively, but can execute other programs. Once the peripheral is ready for data exchange, it can make a service request to the CPU. If the CPU responds to the request, it will temporarily stop the execution of the current program and switch to execute the service program corresponding to the request. After completion, it will continue to execute the original interrupted program.
The advantages of interrupt handling are obvious. It not only saves the CPU the time spent querying peripheral status and waiting for peripherals to be ready, thus improving CPU efficiency, but also meets the real-time requirements of peripherals. However, it requires assigning an interrupt request number and a corresponding interrupt service routine to each I/O device. In addition, an interrupt controller (I/O interface chip) is needed to manage the interrupt requests from I/O devices, such as setting interrupt masking and interrupt request priorities.
In addition, the disadvantage of interrupt handling is that an interrupt must be triggered for each character transmitted, the interrupt controller must be started, and the context must be saved and restored so that the original program can continue to execute. This requires a lot of work, so if a large amount of data exchange is needed, the system performance will be very low.
(3) DMA (Direct Memory Access) transfer method
One of the most obvious features of DMA is that it uses a dedicated controller instead of software to control the data exchange between memory and peripherals, without the need for CPU intervention, which greatly improves the CPU's efficiency.
Before performing DMA data transfer, the DMA controller requests bus control from the CPU. If the CPU allows it, it relinquishes control. Therefore, during data exchange, the DMA controller holds bus control, and after the transfer is completed, the DMA controller returns bus control to the CPU.
(4) Unconditional transmission method
(5) I/O channel mode
(6) I/O processor mode
Industrial control networks
Industrial control networks are typically local area networks (LANs), with a coverage area generally within a few kilometers. They connect monitoring and control equipment distributed around production facilities to form automated systems with varying functions. Control networks are ubiquitous in factory production workshops, assembly lines, greenhouses, grain depots, dams, tunnels, various traffic pipeline systems, buildings, military facilities, fire protection systems, environmental monitoring systems, and residential buildings.
Nodes of industrial control networks
The nodes in industrial control networks are mostly measuring devices with computing and communication capabilities. They may have embedded CPUs, but their functions are relatively simple, and their computing power may be far less than that of a regular PC. They also lack human-computer interaction interfaces such as keyboards and displays. Some even lack CPUs and microcontrollers, only having simple communication interfaces, such as limit switches, inductive switches, and various other switches, as well as various sensors and transmitters for photoelectric, temperature, pressure, flow, and level measurements, and various data acquisition devices, etc.
Controlling network tasks and working environment
Industrial control networks face strong electromagnetic interference from industrial production, various mechanical vibrations, and harsh outdoor working environments, requiring them to adapt to these challenging conditions. Furthermore, the wide variety of automated control equipment makes interconnection and interoperability of control networks extremely difficult.
Real-time requirements of control networks
The control network must meet the real-time requirements of control. Real-time control often requires accurate and timed updates of data for certain variables, and the control action must be completed within a certain time limit, or the relevant control actions must be completed in the order specified by the event [1].
Field interface classification
Based on these characteristics of the control network, the various interfaces must meet the requirements of the control network. Currently, there are four types of interfaces in industrial fields [2]:
(1) Platform-related common protocol: OPC/DDE
OPC was proposed to standardize the software interfaces between devices and applications from different vendors, simplifying data exchange. As a result, it allows users to access process control software components that can be freely combined and used, independent of specific programming languages and environments.
(2) Platform-independent communication protocols: ModBus, ProfiBus
Modbus is a universal language used in electronic controllers. Through this protocol, controllers can communicate with each other and with other devices via networks (such as Ethernet). It has become a universal industrial standard. With it, control devices from different manufacturers can be connected to form industrial networks for centralized monitoring. PROFIBUS is an international, open, and manufacturer-independent fieldbus standard. PROFIBUS transmission speeds are selectable from 9.6 kbaud to 12 Mbaud, and all devices connected to the bus should be set to the same speed when the bus system is started. It is widely used in manufacturing automation, process industry automation, and automation in other fields such as building, transportation, and power. PROFIBUS is a fieldbus technology used for factory automation shop floor level monitoring and field device-level data communication and control.
(3) Platform-independent proprietary protocols: most DCS protocols and industrial Ethernet protocols
(4) Special protocols: Protocols obtained through special methods such as programming ports and printing ports.
There are many types of protocols for industrial transmission communication, mainly due to historical legacy and artificial monopoly. Although there are still a large number of fieldbus standards, none of them are more viable than industrial Ethernet [2].
Summary of various fieldbuses/protocols/interfaces [3]
Industrial automation bus/protocol/interface name
ASI
The AS-interface, a sensor/actuator bus for lower-level control, connects sensors and actuators to the upper-level control layer, offering simple and economical wiring. AS-interface complies with international standards EN50295 and IEC62026-2. AS-Interface (AS-i = Actuator/Sensor Interface) is a fieldbus communication solution for connecting actuators and sensors.
[Compilation] ASI Interface/Protocol/Specification
BACnet == Building Automation Network Data Communication Protocol
BuildingAutomationControlNetwork
[Summary] Communication Protocols for Building Automation in Industrial Automation: BACnet
CANopen
The multi-master bus used in actuator/sensor applications makes efficient use of bus bandwidth, enabling CANopen to achieve short system response times even at relatively low data transmission rates. The main advantages of the CAN bus include: high data security and the ability to retain multi-master capabilities.
CC-Link
CC-Link (Control & Communication Link), a fieldbus primarily targeting the Asian market, is an open bus system used for communication between the control level and the fieldbus level, with applications mainly in the Asian region.
ControlNet
ControlNet is a standardized fieldbus system that allows cyclic and acyclic data to be exchanged simultaneously without affecting each other.
DALI
The DALI standard (IEC 60929), a communication standard in the field of building automation, is a cross-vendor protocol designed to ensure interoperability of electronic ballasts in lighting applications. This new standard replaces the 1-10V dimmer interface. Digital Addressable Lighting Interface (DALI) is a building automation standard for the digital control of electronic ballasts. DALI is used as a subsystem, such as for lighting, blinds, or temperature control, and can communicate directly with the building management system.
[Summary] Lighting Interface in Building Automation: Industrial Automation Bus System - DALI
DeviceNet
DeviceNet, a device bus system using CAN technology, is a sensor/actuator bus system that originated in the United States but is now increasingly used in Europe and Asia. DeviceNet is based on the CAN (Controller Area Network) bus.
DMX
EIB
The EIB (European Installation Bus) communication standard in the field of building automation is a bus system used for building cabling and is widely used mainly in Europe.
EnOcean
Maintenance-free, battery-free, and wiring-free, this is a wireless technology. It's primarily used in building automation: different device modules (such as a light switch) embed EnOcean support, allowing the device to be controlled via a controller.
[Summary] Wireless Protocols for Building Automation in Industrial Automation: EnOcean
EtherCAT == High-speed real-time Ethernet fieldbus
EtherCAT (Ethernet Control Automation Technology) is an Ethernet solution for industrial automation, characterized by its superior performance and ease of operation.
EtherNet/IP
The industrial Ethernet solution Ethernet/IP from ODVA is an industrial Ethernet standard developed by ODVA (Open Device Network Vendor Association), which is based on the Ethernet TCP/IP and UDP/IP standards.
Ethernet TCP/IP
Ethernet, a network bus, is an important standard in the office environment. Many of the advantages of Ethernet, such as high transmission speed, simple integration with existing networks, wide service range, and multiple interface types, are fully reflected in Beckhoff's Ethernet products.
FIAS
FIAS == Fidelio Interface and Application Specification == (estimated) **Fidelio Interface and Application Specification** FIAS (Fidelio Interface and Application Specification) is the interface for the world's leading hotel management software.
[Summary] Interfaces of Industrial Automation, Building Automation, and Hotel Management Software: FIAS and Fidelio
Fipio
The FIP fieldbus conforms to the European standard EN50170 and is supported by independent manufacturers from the WorldFIP fieldbus organization. Through periodic and aperiodic variable and information exchange, bus performance can be tuned according to specific applications.
IEEE1588
InterBus
Interbus, which entered the market in 1987, is a fieldbus system characterized by easy configuration, high speed, and high reliability. This master/slave system's shift register communication protocol can improve the efficiency of cyclic communication.
[Summary] Fieldbus: InterBus
IO-Link
IO-Link perfectly standardizes the different interfaces required for control systems and tooling. IO-Link uses an economical point-to-point connection method to connect sensors and actuators to the control layer. As an open interface, IO-Link can be integrated into all commonly used fieldbus systems. IO-Link systems consist of IO-Link devices, such as sensors, actuators, or a combination of both. They employ traditional three-wire connection technology.
[Compiled] IO-Link
LightBus == High-speed fiber optic fieldbus
Interference-free fieldbus communication for high-speed mechanical control is a fiber optic bus system developed by Beckhoff. It is highly mature and its main features include: strong resistance to electromagnetic interference, easy installation, very fast data transmission speed, cyclicity, and high determinism.
LON
The communication standard LON (Local Operating Network) in the field of building automation is a communication system capable of multiple network connections for distributed applications, primarily used in the field of commercial building automation.
Modbus
Modbus is an open fieldbus protocol based on a master/slave architecture. It has gained widespread acceptance because it can be easily implemented on all types of serial interfaces.
MP-Bus
[Compilation] MP-Bus
Profibus
PROFIBUS USDP/FMS conforms to the European standard EN50170. PROFIBUS, as a high-speed bus, is widely used in distributed peripheral devices (PROFIBUS USDP). In addition to PROFIBUS USDP and FMS, Beckhoff also supports the PROFIBUSMC protocol for drive communication.
PROFINET
PROFINET, an industrial Ethernet solution from PNO (PROFIBUS User Organization), is an open industrial Ethernet standard developed by PNO, while internationally recognized IT standards such as TCP/IP are used to achieve communication.
RS232/RS485
The most economical solutions are network RS232 and RS485, which are 'traditional' serial interfaces that have been widely used for a long time. Beckhoff's RS485/RS232 I/O modules use a simple and easy-to-implement serial communication protocol.
SERCOSIII
Originating from the field of driver engineering, SERCOS was originally developed as a high-speed fiber optic bus system for drivers. Thanks to Beckhoff's development of the SERCOS bus coupler, some of the advantages of the SERCOS bus (such as high data transfer rates and short cycle times) can now also be used for I/O peripherals.
USB
The high-speed USB interface for laboratory use has become the standard interface in the PC technology field. Due to its high transmission rate and flexible topology (through integrated hubs), coupled with Beckhoff's USB bus coupler, this system can replace fieldbus in short-distance applications.
for example:
Beckhoff's CX9000 can be connected to multiple buses via a bus coupler:
EtherCAT can integrate many fieldbuses:
Similar physical diagrams of other buses are shown below:
As you can see, the buses that have been extended above are:
Profibus
Profinet
CANopen
DeviceNet
